1. Field of the Invention
The present invention relates to the field of variable gain amplifiers.
2. Prior Art
Various types of variable gain amplifiers are well known in the prior art. Such amplifiers are frequently used for amplifying signals of varying signal strength to provide an output signal of a predetermined signal strength for direct use or for further signal processing in the system. Of particular importance for such applications is the range of gain available, and the noise added to the signal by the variable gain amplifier. Since each stage amplifies the noise imposed by the prior stages as well as the signal, it is particularly important that the variable gain amplifier have a low noise input stage commensurate with the overall gain of the amplifier, as merely increasing the gain range of a variable gain amplifier without a corresponding control of the noise of the amplifier will only provide an output for the highest gains dominated by the noise of the input stage of the amplifier.
Also important in many applications is the controllability of the gain of a variable gain amplifier. In the least demanding applications, a reasonably well behaved, monatonic variation in gain with gain control voltage is adequate. In the more demanding applications however, a predefined variation of gain with gain control voltage, repeatable over the operating temperature range of the variable gain amplifier, such as a log-linear variation, is a necessity. The preferred embodiment of the present invention provides a variable amplifier having low noise, a wide variable gain range and a predefined, stable over temperature, gain variation over the full range of gain control and temperature variations.